At the present, a variety of surface-finished steel materials are manufactured in the steel industry and today almost 80% of the fine sheet metal products in Germany are supplied in a surface-finished form. For the production of products, these fine sheet metal products are processed further, so that a wide variety of different metallic materials or a wide variety of combinations of metallic base materials and surface materials may be present in one part and, to meet certain product requirements, must be present. In further processing, especially of surface-finished steel plate, the material is cut to size, shaped and joined by welding or adhesive bonding methods. These processing operations are typical to a great extent of vehicle body production in the automobile industry, where mainly galvanized steel plate from the coil coating industry is processed further and joined to ungalvanized steel plate and/or aluminum plate, for example. Vehicle bodies consist of a multitude of sheet metal parts joined together by spot welding.
From this variety of combinations of metallic sheet materials in one part and the primary use of surface-finished steel plates, special requirements are derived for corrosion protection, which must be capable of reducing the consequences of bimetal corrosion as well as corrosion at cut edges. Although metallic zinc coatings applied to steel plate electrolytically or in a melt-dip process impart a cathodic protective effect, which prevents active dissolution of the more noble core material at cut edges and mechanically induced damage to the zinc coating, it is equally important to reduce the corrosion rate per se to ensure the material properties of the core material. Requirements of the corrosion prevention coating, consisting of at least one inorganic conversion layer and one organic barrier layer are high accordingly.
At cut edges and at any damage to the zinc coating caused by processing or other influences, the galvanic coupling between the core material and the metallic coating produces an active unhindered local dissolution of the coating material, which in turn constitutes an activation step for corrosive delamination of the organic barrier layer. The phenomenon of debonding of paint or “blistering” is observed especially at cut edges, where unhindered corrosion of the less noble coating material occurs. The same thing is also true in principle for the locations on a part where different metallic materials are joined together directly by joining techniques. Local activation of such a “defect” (cut edge, damage to the metal coating, spot welds) and thus corrosive debonding of paint emanating from these “defects” are all the more pronounced, the greater the electric potential difference between the metals in direct contact. Equally good results with regard to paint adhesion at cut edges are offered by steel plate with zinc coatings alloyed with more noble metals, e.g., iron-alloyed zinc coatings (Galvannealed steel).
The producers of steel plate have been relying to an increasing extent on integrating other corrosion coatings, in particular paint coatings, into the plate mill, in addition to surface finishing with metallic coatings, so there is an increased demand for anticorrosion treatments capable of effectively preventing the problems associated with corrosion of cut edges and contact corrosion in adhesion of paint there and also in the processing industry, in particular in automotive manufacturing.
Various pretreatments which address the problem of edge protection are known in the prior art. The essential strategy being pursued here is to improve adhesion of the organic barrier layer to the surface-finished steel plate.
Unexamined German Patent DE 19733972, which describes a method of alkaline passivating pretreatment of galvanized and alloy-galvanized steel surfaces in metal plate mills, is to be considered the most proximate prior art. In this method, the surface-finished steel sheet is brought in contact with an alkaline treatment agent containing magnesium ions, iron(III) ions and a complexing agent. The zinc surface is passivated, forming the anticorrosion layer, at the predefined pH of more than 9.5. According to the teaching of DE 19733972, a surface passivated in this way offers paint adhesion comparable to that of methods using nickel and cobalt. Optionally this pretreatment for improving corrosion protection may be followed by other treatment steps, such as a chromium-free post-passivation, before applying the paint system. It has nevertheless been found that this pretreatment system is unable to satisfactorily suppress the debonding of paint caused by corrosion at cut edges.